A high intensity light source

ABSTRACT

A high intensity light source produces the principal portion of its light energy output substantially within the spectral range of 4,600 A to 5,000 A. A sealed envelope of material substantially transparent to light energy within the aforementioned range contains electrically conductive electrodes, a suitable starter gas, preferably of an inert character, and also an amount of elemental metal. The electrodes have external terminals which, upon connection to a suitable source of electrical energy, generate an initial electrical discharge producing sufficient heat to cause sublimination of the metal so that it is converted to its vapor state. The metal is chosen to have a sufficiently high vapor pressure so that it will sublimate in response to a temperature of the order of approximately 800*C. One such metal is zinc, for example. Moreover, the amount of elemental metal contained within the envelope is such as to develop a sufficient pressure within the envelope for producing the principal light energy output within the desired stated spectral range. In the preferred embodiment the starter gas is an inert gas and the spacing between the electrodes is a relatively short gap ensuring an extremely high spectral radiance.

[ 1 Sept. ll, I973 A HIGH INTENSITY LIGHT SOURCE [75] Inventors: MyerGeller; Daniel E. Altman;

Glidden J. Barstow, all of San Diego,

Calif. 92106 57 ABSTRACT A high intensity light source produces theprincipal portion of its light energy output substantially within thespectral range of 4,600 A to 5,000 A. A sealed en- [73] Assignee; TheUnited States of Ameri a a velope of material substantially transparentto light enrepresented by the Se retary of the ergy within theaforementioned range contains electri- Navy, Washington, DC. callyconductive electrodes, a suitable starter gas, preferably of an inertcharacter, and also an amount of ele- 2 Flled 1971 mental metal. Theelectrodes have external terminals [21] Appl. No.: 192,857 which, uponconnection to a suitable source of electrical energy, generate aninitial electrical discharge pro ducing sufficient heat to causesublimination of the [2%] 31322565134227 metal so that it is convertedto its vapor Stata The 5 metal is chosen to have a sufficiently highvapor pres- 1 le 0 sure so that it will sublimate in response to atemperature of the order of approximately 800C. One such [56] ReferencesC'ted metal is zinc, for example. Moreover, the amount of el- UNITEDSTATES PATENTS emental metal contained within the envelope is such as3,363,134 1/1968 Johnson 313/225 to develop a sufficient pressure withinthe envelope for FOREIGN PATENTS OR APPLICATIONS producing the principallight energy output within the 10 desired stated spectral range. In thepreferred embodi- 7,764 6/1939 Australia 313 225 mem the Starter gas isan inert gas and the Spacing tween the electrodes is a relatively shortgap ensuring Primary Examiner-Roy Lake an extremel hi h s ectralradiance. Assistant Examiner-Darwin R. Hostetter y g p Att0rneyR. S.Sciascia-et al. 8 Claims 5 Drawing Figures TINTENSITY I I I l I I I I II l I I I I I I I I 4600 4700 4800 4900 5000 5500 6000 6100 6200 63006400 PATENTED SEPI I I975 SHEU 1 OF 2 IOOM 200 M ATTENUATION COEFFICIENTWAVELENGTH 3.

FIG. 1

TI A3 VAPOR PRESSURE (TORR) IO ll IOOO-r E Q I2 I50- FIG. 2 IO 1 I I I ii MYER GELLER 200 600 I000 BY DANIEL E. ALTMAN TEMPERATURE 0 1 GLI%TENJ. BARSTOW FIG. 5 MF 1 A IIIGII INTENSITY LIGHT SOURCE CROSS REFERENCETO RELATED APPLICATION The subject matter of the present invention isgenerally related to that of copending U.S. Pat. application, Ser. No.158,330, titled Light Source for Use in Deep Ocean Water, filed June 30,1971 in the names of Myer Geller, Daniel E. Altman, and Glidden J.Barstow.

BACKGROUND .OF THE INVENTION Underwateroptical systems such as areemployed for viewing, surveillance, range gating, communication,

etc., are best implemented with light sources producing. high energyoutputs generally in the blue portion of the light spectrum and moreparticularly between the spectral range of 4,600 A and 4,900 Awhereocean waters impose minimum attenuation and thus pennit the greatesttransmissivity of light energy.

Prior art light sources presently employed in deep ocean underwateroptical systems include a frequency doubled yttrium aluminum, garnet,neodymium doped, YAG(ND) laser radiating at about 5,300 A. This type ofsource is, however; unfortunately relatively inefficient inasmuch as itspeak energy outputs occur at wavelengths which are fairly close to thedesired spectral range but are nonetheless not within the maximumtransmissivity of ocean water which lies substantially between thewavelengths 4,600 A and 4,900 A. Moreover, this type of light sourcerequires very careful control of both its operative temperature and theoptical alignment of its doubler.

The argon ion laseris another type of prior art light .source which hasbeen used in underwater optical systems. The argon ion laser has twovery strong lines of peak energy output at approximately 4,880 A and5,140 A. However, it is also relatively very inefficient because theprincipal portion of its output energy is not within the spectral rangewhere highest transmissivity occurs in ocean waters. Additionally, inthe present state of the art output powers of more than five watts arenot readily available from such a device.

Pulsed xenon lasers have also been employed experimentally in underwateroptical systems, producing two strong outputs at about 4,954 A and 5,005A which are reasonably close to the spectral range of maximumtransmissivity or minimum attenuation in deep ocean waters. However,pulsed xenon lasers are still in a state of development and have thedisadvantage of being relatively inefficient in respect of the amount ofinput power required to produce a minimally usable amount of lightoutput energy.

A number of non-coherent light sources' are also available such as thetungsten incadescent lamps and the mercury or thallium iodide vapor arcdischarge lamps. Unfortunately, however, these type of prior art lightsources develop most of their radiant energy outside the limitedspectral range within which deep ocean water has the highest degree oftransmissivity.

SUMMARY OF THE INVENTION The present invention comprises a highlydesirable and efficient light source for underwater optical systems,producing its principal light energy output substantially within thespectral range of 4,600 A to 5,000 A where the" greatest degree oftransmissivity may be realized. That spectral range (because of itsmaximum transmissivity and minimum attenuation of light energy throughdeep ocean water) is sometimes referred to as the water window."

In its preferred embodiment the present invention comprises a sealedenvelope fabricated of material which is substantially transparent tolight energy within the aforementioned spectral range such as high gradequartz, for example. Sealed within the'envelope are two electrodes whichare preferably spaced and positioned relative to each other so as toprovide a short arc discharge through an appropriate starter gas. Thestarter gas is preferably an inert gas such as xenon which will insureminimum deterioration of the electrodes due to sputtering, corrosion andother deleterious effects.

The envelope also contains an amount of an elemental metal chosen forthe physical characteristics it exhibits under certain controlledconditions which will be explained more fully hereinafter.

The electrodes extend from within the sealed envelope to electricallyconductive external terminals suitable for connection to a source ofelectrical energy. Upon the application of electrical energy to theexternal terminals, the starter gas within the sealed envelope ionizesand creates a short arc between the relatively closely spacedelectrodes.

The elemental metal contained within the envelope is chosen from one ofseveral-metals having a sufficiently high vapor pressure to causesublimination into a vapor state in response to the heat generated bythe initial discharge of the ionized starter gas between the closelyspaced electrodes; most importantly, the elemental metal must possessenergy levels such that strong transitions occur in the water window"and that the transitions dominate the output spectrum; the amount ofsuch elemental metal within the envelope must be such that it willdevelop a pressure within the envelope for producing the principal lightenergy output of the assem'bly'of the light source within the desiredspectral range, i.e., substantially the water window.

In accordance with the concept of the present invention, a metal such aszinc or cadmium may be chosen because of its high vapor pressurecharacteristics which will enable it to subliminate and be transformedto its vapor state at a temperature of the order of approximately 800C.Additionally, the amount of such chosen elemental metal contained withinthe sealed envelope is sufficient to develop a significant vaporpressure which may, for example, be of the order of one atmosphere in atypical embodiment, enabling the light source assembly of the presentinvention to produce principal light energy output which differssignificantly from the usual principal light energy output of a vaporarc lampemploying the same metal but operating at relatively low vaporpressure. A low vapor pressure lamp employing the same elemental metalwithin a sealed envelope produces its principal light source atwavelengths outside the desired spectral range of water window." Theconcept of the present invention, when practiced under the prescribedhigh pressure conditions and employing a 'metal chosen for its physicalcharacteristics as taught by the present invention develops a principalamount of light energy output falling well within the desired spectralrange of water window."

Accordingly, it is a primary object of the present invention to providean improved metal vapor arc light 3 source for generating its principallight energy output within the spectral range of wavelengths having thegreatest transmissivity in ocean waters.

Another most important object of the present invention is to providesuch a lamp which is operative at relatively modest temperatures.

Yet another important object of the present invention is to provide sucha lamp wherein a metal is chosen which has a high vapor pressureenabling the metal to sublimate to its vapor state in response to theheat of the are developed by an inert starter gas.

A further object of the present invention is to provide such a lightsource which will produce a principal portion of its light output energywithin the desired spectral range when operated at a temperature belowthat which will cause the devitrification of quartz.

Another object of the present invention is to provide a light sourceproducing its principal light energy output within the designed spectralrange operative in a short are configuration for producing extremelyhigh spectral radiance or brightness per unit wavelength.

These and other features, objects, and advantages of the presentinvention will be better appreciated from an understanding of theoperative principles of a preferred embodiment-as described hereinafterand as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a graphic illustration of the attenuation of differentwavelengths .of light at several different depths of ocean water;

FIG. 2' is an illustration of a preferred embodiment of the presentinvention;

FIG. 3 is a graphical illustration of the variation in vapor pressuredeveloped by the elemental metal zinc in its vapor state relative totemperature;

FIG. 4 is a graphical illustration of the spectrum of light energyoutput developed by a light source embodying the present invention andemploying elemental zinc;

FIG. 5 is a graphical-illustration of the spectrum of light energyoutput developed by a light source embodying the present invention andemploying elemental cadmium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 graphically illustratesthe variation of attenuation of light ocean water at different depthsand also at different wavelengths of light. In the illustration of FIG.1 the attenuation of light at different depths of meters, 100 meters,200 meters, and 2,700 meters is illustrated by a series of curves whichencompass varying wavelengths generally from approximately 4,000 A to6,000 A. It is evident that less attenuation of light generally occursat greater depths. Additionally, however, it is also evident from theillustrations of FIG. 1 that minimum attenuation or greatesttransmissivity of light is realized at all depths within the spectralrange of approximately 4,500 A to 5,000 A.

Accordingly, light sources for use in ocean waters desirably produce theprincipal portion of their light energy output within the range ofgreatest transmissivity which may be referred to as the water window."It is also desirable that a light source for use in an ocean environmentbe as simple in its concept and operation as 4" is possibleandconsistent with the requirement that it be efficient in developing itslight energy output within the desired spectral range to a high degreeof spectral radiance.

FIG. 2 illustrates a preferred embodiment of the present invention whichincludes a sealed envelope l0 fabricated of a material which issubstantially transparent to light energy within the desired spectralrange, such as quartz, for example, and having two electrodes 11 and 12sealed within its interior. The electrically conductive electrodes 11and 12 have external terminals 13 and 14 adapted for connection to asource of electrical energy.

Within the interior of the sealed envelope 10 there is included asuitable starter gas preferably of a inert character, such as argon orxenon, and also an amount of a metal in its elemental form. The metal'may be supported on the interior walls of the sealed envelope. Themetal is chosen for a relatively high vapor pressure characteristic suchas will enable the metal to be sublimated to its vapor state in responseto the heat developed by the initial discharge of the conductive arebetween the two electrodes within the envelope upon the application of asuitable source of electrical energy to their external terminals.

As is well known in the art, the starter gas becomes ionized and forms aconductive arc to start the operation of the light source. The heat ofthe arc causes the subsequent sublimation of the elemental metal whichchanges directly to its vapor state.

Additionally, the metal must have strong electronic radiationtransitions in the 4,600 A 5,000 A region. The concept of the presentinvention teaches that the metal must be chosen to have certain requiredcharacteristics. .For example, the energy level structure of atomicneutral zinc exhibits transitions of interest including three principallines where transitions are of the type 5 S 4P. The atomic parameters ofthese three lines are:

4680.1 A has. gA 5.8 X 10 sec 4722.2 A has gA =15 10 4810.5 A has gA =21X 10 When a starter gas such as argon or xenon is used, the 5 8 level islower in energy than the lowest excited state of either argon or xenon.Thus, as the zinc metal evaporates and enters the discharge, theradiation from this multiplet dominates and the spectrum of the startergas essentially disappears.

The amount of elemental metal which is sealed withinthe envelope 10 iscarefully chosen so that a constant state of vapor saturation willprevail and a relatively high vapor pressure will be developed withinthe sealed interior of the light source when it reaches its fullyoperative state. FIG. 3 illustrates the variation in vapor pressure ofzinc in its elemental form responsive to a range of temperatures undervapor saturation conditions. From the illustration of FIG. 3 it can beseen that when elemental zinc is heated to a temperature ofapproximately 900C it will develop a vapor pressure of nearly oneatmosphere. In accordance with the concept of the present invention, asufficient amount of the elemental metal is sealed within the interiorof the light source for developing a relatively high pressure at afairly modest temperature, with the result that the prin cipal portionof the light energy output developed and generated is not onlydistinctly different from the spectral range of light energy outputdeveloped in a metal are vapor lamp employing zinc at a relatively lowpressure, but also desirably falls within the range of approximately4,600 A to 5,000'A which has been referred to as the water window."

Moreover, the electrodes iland 12 are preferably spaced in a short arcconfiguration providing high spectral radiance, i.e., brightness perunit wavelength. A preferred embodiment of the present inventionsubstantially as illustrated in FIG. 2 with a power input of 800 watts,and employing the short arc" configuration having electrodes spaced at-0.10 inches, emitted 13 watts within the desired spectral rangecomprising the water window. Thus, the light'source of the presentinvention provides an extremely high spectral radiance and produces theprincipal portion of its light energy output within the desired spectralrange.

FIG. 4 is a graphical illustration of actual test data developed fromthe operation of a light source embodying the concepts of the presentinvention, using argon as the starter gas, and zinc as the elementalmetal. It will be seen that extremely strong peaks of light energyoutput are developed at approximately 4,680 A, 4,720 A, and 4,810 A. Afourth strong output of energy is developed at approximately 6,360 Awhich is outside the most desirable spectral range. However, the greaterportion by far of the light energy output of the light sourceconstructed and operated in accordance with the concept and teaching ofthe present invention is clearly within the desired spectral rangecomprising the water window. The conventional low pressure zinc arcvapor lamp produces distinctly different spectral outputs spread over anextremely broad spectral range so that its employment as an acceptablyefficient underwater light source is for all practical purposes almostwholly ineffective. The highly desirable results produced by the conceptof the present invention is achieved through the use of an amount ofelemental metal in sufficient quantity to ensure saturation condi-'tions at the operative temperatures and develop relatively high vaporpressure which brings about the principal output of light energy withinthe desired spectral range.

Another suitable elemental metal which may be emtemperatures are wellbelow the temperatures at which quartz will devitrify, exhibitingundesirable deterioration effects'such as clouding, etc. Many prior artknown' lamps which attempt to achieve the same ob- 5 jects andpurposesas the present invention are required ployed in accordance withthe concept of the present invention is cadmium and the same generalteachings are equally applicable in the use of cadmium as-in thepreviously described employment of zinc.

FIG. 5 graphically-illustrates a light energy output of a lampfabricated and operated in accordance with the concepts of the presentinvention and employing cadmium as the elemental metal. In theillustration of FIG. 5 it may readily been seen that three principalpeaks of light energy output are developed at approximately 4,675 A,4,797 A, and 5,085 A, with another single peak of high energy outputdeveloped in the general range of approximately 6,450 A. Thus, the useof cadmium in a light source embodying the present invention tiveefficiency and the production of the principal portion of its usefulenergy output within the desired spectral range when the light source isoperated at a relatively modest temperature. Desirably, these operativeto operate at significantly higher temperatures, sharply limiting thelife of quartz envelopes, or requiring the employment ofdifferentmaterials other than quartz which are not subject to deleterious effectsat such high temperatures.

Another and most important advantage of the present invention is that inits preferred embodiment it is operative in a short arc" configurationwhich provides a concentration of light energy output to developextremely high spectral radiance not readily achievable with known priorart light-sources capable of producing significant amounts of outputenergy within the desired spectral range and operating in the continuousmode.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

L'A high intensity light source producing its principal light energyoutput substantially within a predetermined spectral range comprising:

a sealed envelope of material substantially transparent to light energywithin said spectral range;

electrically conductive electrodes communicating with the interior ofsaid envelope, forming a short gap therebetween, and having externalterminals for connection to a source of electrical energy, said shortgap being dimensioned and configured relative to the current capacity ofsaid source of electrical energy for producing a high .power density am;

a starter gas contained within said sealed envelope for generating aninitial electrical discharge between said electrodes upon the connectionof a source of electrical energy to said terminals; and

a metal contained within said envelope;

said metal having sufficiently high vapor pressure to cause itssublimation in response to the heat generated by said initial discharge,and being present in an amount developing a pressure within saidenvelope of a magnitude for sustaining said high power density are andgenerating its principal light energy output within the spectral rangeof 4,600A to 5,000A.

2. A light source as claimed in claim 1 wherein said metal is initiallysupported on the inner walls of said envelope.

3. A light source as claimed in claim 1 wherein said metal is zinc.

4. A light source as claimed in claim 1 wherein said metal is cadmium.

5. A light source as claimed in claim 1 wherein said starter gas is aninert gas.

6. A light source as claimed in claim 1 wherein said starter gas isargon. I

7. A light source as claimed in claim 1 wherein said starter gas isxenon.

8. A light source as claimed in claim 1 wherein said envelope isfabricated of quartz material.

t! t t 4' 4

2. A light source as claimed in claim 1 wherein said metal is initiallysupported on the inner walls of said envelope.
 3. A light source asclaimed in claim 1 wherein said metal is zinc.
 4. A light source asclaimed in claim 1 wherein said metal is cadmium.
 5. A light source asclaimed in claim 1 wherein said starter gas is an inert gas.
 6. A lightsource as claimed in claim 1 wherein said starter gas is argon.
 7. Alight source as claimed in claim 1 wherein said starter gas is xenon. 8.A light source as claimed in claim 1 wherein said envelope is fabricatedof quartz material.